Method and apparatus for drying a coated paper web or the like

ABSTRACT

Method and apparatus for drying a coated paper web (W) or the like web. The apparatus ( 10 ) comprises sequentially in the running direction of the web—a turning device ( 16 ) arranged on the first side of the web and provided with blow nozzles ( 20 ), with which the running direction of the web to be dried is turned in a non-contacting way, and web drying devices ( 30, 30 ′) arranged on the first and the second sides of the web, in which the web is dried in a non-contacting manner. The apparatus further comprises a counterpart ( 18 ) provided with overpressure nozzles ( 24 ), arranged on the second side of the web at the point of the turning device ( 16 ), for stabilizing the web run by means of blows generating a local overpressure between the web (W) and the carrier surfaces of the overpressure nozzles ( 24 ).

This application is a 371 PCT /FI98/00567 filed Jul. 3, 1998 whichclaims priority from Finnish application 972 878 filed Jul. 7, 1997.

The present invention relates to a method and an apparatus for drying acoated paper web or the like.

From the U.S. Pat. Nos. 5,771,602 and 5,230,165, it has previously beenknown to turn a coated but still undried paper web or the like in anon-contacting way by blows generated by a turning device before the webis actually dried in a non-contacting way by airborne web-dryersarranged on both sides of the web.

From the U.S. Pat. No. 5,230,165, it is also per se known to arrangecurved counterpart provided with underpressure nozzles against theturning device on the other side of the paper web. The purpose is tobegin the drying of the paper web on both sides of the web already atthe curved section of the web. The underpressure nozzles have arelatively limited drying capacity, as drying air is blown to flowprincipally in the direction of the web along the nozzle surface,utilizing the Coanda effect. In the arrangements shown, the moist andstill warm drying air blown from the underpressure nozzles is allowed toflow from the counterpart directly into the machine room surrounding theapparatus, which adversely increases moisture and heat in the machineroom, in addition to the fact that wasting heat as such does not conformto principles of sound energy economy.

Due to the Coanda effect, a static underpressure zone is formed betweenthe nozzles and the web in the nozzle area in the counterparts known perse, principally over the entire nozzle area. The aim is to use thisunderpressure to intensify the pushing effect of the turning device bymeans of suction in the counterpart area. Suction is used to spread theweb outwards in order to stabilize the web on its curved course.However, regarding these arrangements provided with underpressurenozzles, there is a risk that the paper web, should it lacken forexample due to tension variations, contacts the nozzles of thecounterpart whereupon the coating is damaged and/or the web breaks.

SUMMARY OF THE INVENTION

The object of the present invention is thus to provide a new, improvedmethod and an apparatus in which the drawbacks presented above have beenminimized.

It especially is an object of the present invention to provide a methodand an apparatus with which it is possible to achieve an improvedrunnability or controllability of a paper web or the like.

It is further an object of the invention to provide a method and anapparatus which make it possible to lead the paper web or the like moresafely through a slot formed by the turning device and the counterpart.

A further object is to provide a method and an apparatus providing alarger drying capacity of the paper web than before, and thus save spacein the machine room.

It is a further object to provide a method and an apparatus with whichit is possible to decrease the moisture and thermal load in the machineroom and thus simultaneously improve the energy economy of the process.

The in which the running direction of the paper web (W) to be dried isturned in a non-contacting way by using blows generated by blow nozzlesof the turning device and by pad pressure thus generated;

The paper web is dried in a non-contacting way with drying devices.

The apparatus typically comprises a counterpart provided withoverpressure nozzles, the counterpart being arranged at the curvedturning device area on the opposite side of the paper web.

Overpressure nozzles refer here to nozzles the blows of which generate aweb pushing power at all distances from the web. In the knownarrangements described above, the starting point has been the reverse;in them, underpressure nozzles have been used for generating at acertain distance from the web a power opposite to the pushing power inorder to spread the web. With the overpressure nozzles of the invention,it is possible to control the running of the web better and to ensurethat the web stays apart from the nozzles.

In an advantageous arrangement of the invention, the counter-part may beprovided with, for example, Float or Push nozzles disclosed in theapplicant's U.S. Pat. No. 4,384,666. On the other hand, if desired, thecounterpart may also be provided with simple impingment nozzles whichinclude, for example, a perforated plate or one or more slots extendingacross the web, from which air is blown principally directly against theweb.

The overpressure nozzles of the counterpart are advantageously arrangedradially against the blow nozzles of the turning device, i.e. so thatblows from the counterpart are directed against the paper web andagainst the blows from the turning device arranged on the first side ofthe web. Thus the blows, for example from Float overpressure nozzles,generate a local overpressure on both sides of the web between the paperweb and the carrier surfaces of the nozzle, i.e. the nozzle surfaces;with this overpressure, the running of the paper web may be stabilizedand the runnability and controllability of the web may be improved.Impingment nozzles provide the same result, although the pressuregenerated by the impingment nozzles generally is slightly lower than thepressure generated by overpressure nozzles of the Float type.

On the straight run of the paper web arranged after the turning section,i.e. in the drying section, floating nozzles on the opposite sides ofthe paper web are, however, arranged advantageously interlaced with eachother so that the web travels in a sine-wave path between the nozzlesarranged on both sides of the web, which allows an as smooth as possibleweb run. It is naturally possible that also part of the nozzles in theturning section of the web are arranged interlaced with each other.

In the turning section of the device, the running direction of the webmay be turned even 20°-260°, typically 30°-160°.

The actual turning device of the invention, in which the runningdirection of the paper web may be turned 20°-260°, comprises typically3-15 blow nozzles. The counterpart advantageously comprises the samenumber of overpressure nozzles, i.e. 3-15 nozzles. Also the blow nozzlesof the actual turning device are preferably overpressure nozzles.

According to the invention, the pushing nozzles of both the turningdevice and the counterpart are principally so-called Float overpressurenozzles of the applicant. As the nozzles in the turning sectionadditionally are arranged opposite to each other on both sides of theweb, the pushing forces caused by the nozzle flows are directed againsteach other. This generates a local overpressure at the carrier surfaceareas of the nozzles on both sides of the web. The local overpressuresarranged opposite to each other on both sides of the web have astabilizing effect on the web run, and improve the runnability andcontrollability of the web, also in cases of disturbance. Thus thearrangement of the invention provides an optimal configuration ofnozzles as to the control of the web.

With the arrangement of the invention, in which overpressure nozzles,such as Float nozzles, are used in the counterpart instead ofunderpressure nozzles known per se, such as Foil or Pull nozzlesdisclosed in the applicant's patent U.S. Pat. No. 4,247,993, for examplethe important advantage is achieved compared with the known technology,that it is considerably less probable that, due to tension variations,the paper web would contact the nozzles of the counterpart or theturning device, because the overpressure nozzle pushes the web away,while an underpressure nozzle is not necessarily always able to keep theweb away from the nozzle surface.

The turning device of the invention is further advantageously providedwith a control device, increasing the controllability of the turningdevice and making it possible to automatically control the distancebetween the turning device and the web, this control being based on theratio between the supply air pressure of the turning device and the padpressure. In addition, the pressures may be used for automaticallycalculating the tension of the web.

Thereby the control device typically comprises

a pressure sensor arranged in the blow nozzles of the turning device formeasuring the internal pressure P_(SP) of the blow nozzle;

a second pressure sensor arranged between the turning device and thepaper web for measuring the pad pressure P_(KL) between the turningdevice and the paper web; and

a control element with which the values of the various pressure sensorsare combined in order to calculate the distance H between the nozzlesurface of the turning device and the paper web and/or in order toadjust it to a desired level.

For the calculation of web tension,

a third pressure sensor arranged between the counterpart and the paperweb for measuring the pressure P_(VK) between the counterpart and thepaper web additionally needed.

The distance H between the nozzle surface of the turning device and thepaper web is, within the typical range of 0-30 mm, derived from theformula: $H = {{a\frac{P_{SP}}{P_{KL}}} + b}$

in which

H is the distance (mm) between the nozzle surface and the paper web;

P_(SP) is the internal pressure (Pa) of the blow nozzles;

P_(KL) is the is the pad pressure (Pa) of the turning device, i.e. thepressure between the turning device and the paper web, measured in theturning device in the free space between the nozzles;

a is the amplification coefficient for the machine;

b is the difference variable for the machine.

The pad pressure refers to overpressure in the turning device, generatedinto the turning device, as a box or a similar structure arranged aroundit restricts the discharge of blowing air from the turning device. Witha certain turning device structure, the pad pressure is principallydependent on the amount of air led to the turning device, the pressureprevailing in the counterpart, and the tension of the web. The padpressure is measured in the free space between the nozzles of theturning device.

The distance between the carrier surface of the nozzles and the paperweb is generally controlled either by adjusting the operating speed ofthe blower blowing air to the blow nozzles of the turning device, or bya guide vane adjuster so that, by controlling the air supply in thisway, also the nozzle pressure P_(SP) of the blow nozzles, and thus alsothe distance of the web from the nozzles, is controlled.

The automatic adjustment of the distance between the carrier surface ofthe turning device nozzles and the paper web is in practice carried outso that the internal pressure P_(SP) of the nozzle of the turning deviceand the pad pressure P_(KL) between the paper web and the turning deviceare measured automatically by two pressure sensors, whereafter thedistance of the web from the nozzle surface is automatically calculatedwith the help of the ratio between the internal pressure in the nozzle(nozzle pressure) and the pad pressure, using the above mentionedformula. When necessary, this ratio may be corrected by adjusting thesupply of blowing air so that the distance of the web from the nozzlesurface remains at a desired level. The adjustment may be automatic, inwhich case the aim is usually to maintain the distance constant bykeeping the ratio between the nozzle pressure and pad pressure constant.

The web run may thus be corrected with the pressure adjustment, forexample, in a case in which the paper web is drawn away from the nozzlesurface due to the decrease in web tension. As the web tensiondecreases, the pad pressure of the turning device decreases and theratio between the nozzle pressure P_(SP) and the pad pressure P_(KL)increases. By reducing the supply of air to the nozzles, for example, byreducing the operating speed of the blower or by adjusting the guidevanes, the nozzle pressure may thus be automatically reduced whereuponthe ratio of the nozzle pressure and the pad pressure, and thus also thedistance of the web from the nozzle surface, decreases.

Besides the web distance, also the paper web tension T may automaticallybe monitored on the basis of values from the pressure sensors, using thefollowing formula

 T=C*[P_(KL)(r+h)−k_(VK)P_(VK)(r+h)+Mv²]

in which

C is the amplification coefficient relating to the machine in questionwithin the range of 0.7-1.4, typically 1.0;

P_(KL) is the pad pressure (Pa) for the turning device, i.e. thepressure between the turning device and the paper web, measured in theturning device in the free space between the nozzles;

P_(VK) is the pressure (Pa) in the counterpart, measured in the freespace between the nozzles;

k_(VK) is a parameter constant machine in question within the range of0.6-1, typically 0.8;

r is the radius (m) of the turning device;

h is the distance (m) between the nozzle surface of the turning deviceand the paper web;

T is the tension (N/m) of the paper web;

M is the grammage (kg/m²) of the paper web;

v is the speed (m/s) of the paper web.

The calculated tension value may be used for controlling the tensionadjustment. A static pressure P_(VK) deviating from the atmosphericpressure may be generated between the web and the counterpart, which isdependent on the running mode, and on the supply and discharge of air;this pressure may be above or below the atmospheric pressure, in whichcase it has to be taken into account when calculating the tension. Itmay be mentioned that the pressures given in this application generallyrefer to pressures in relation to the atmospheric pressure, unlessstated otherwise.

The pressure in the counterpart also affects the pad pressure betweenthe turning device and the web. By adjusting the pressure in thecounterpart, within the range from over-pressure to underpressure, theweb run may thus also be controlled from the counterpart side.

When desired, the overpressure nozzles of the counterpart, as well asthe blow nozzles of the turning device, may be used for blowing hot aironto the paper web, the temperature of air being 100-450° C., preferably150-400° C., and the speed of air 20-100 m/s, preferably 40-80 m/s sothat the paper web may efficiently be dried from both sides of the webalready in the turning section. In the turning section, a more efficientdrying is achieved by overpressure nozzles than by underpressurenozzles, due to better nozzle geometry. With the over-pressure nozzles,a bigger heat transmission coefficient may be achieved than withunderpressure nozzles, due e.g. to the turbulence of the air flow beingdischarged from them.

As the web is dried after the turning device by using airborne web-dryerunits provided with exhaust air channels, it is also advantageous todischarge hot blowing air from the turning device and the counterpartthrough the exhaust air channels of the airborne web-dryer units. Thusmoist and hot air is not led from the turning section to the machineroom to increase its moisture and thermal load.

The turning device on the first side of the paper web and the airborneweb-dryer unit following it may advantageously be covered with a commonhousing structure. Likewise, the counterpart on the opposite side of theweb and the airborne web-dryer unit following it may advantageously becovered with a common housing structure.

As a summary it may be said that the following advantages are achievedwith the two-sided turning device of the invention, i.e. a turningdevice provided with a counterpart of the invention:

good runnability and controllability of the web, also automatically;

reliable follow-up of the web tension;

non-contacting travel of the web;

higher web speed possible;

more efficient drying possible;

better energy economy, due to the reduction of moisture and thermal loadin the machine room, as the free draws decrease and the recovery ofexhaust air becomes possible, and due to the recycling of exhaust airfrom the airborne web-dryer to the turning device;

saving of space, due to the better vaporization efficiency in thelongitudinal direction of the web, because it is possible to maintainthe performance characteristics typical of the airborne web-dryer in thecounterpart, e.g. blowing speed 40-80 m/s, temperature 200-400° C., andvaporization 60-180 kg/m²h.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is next described in more detail referring to the encloseddrawings in which

FIG. 1 is a schematic, partially vertical section of a two-sided turningdevice of the invention;

FIG. 2 is a schematic view of a control system of the two-sided turningdevice of FIG. 1; and

FIG. 3 is a schematic, enlarged view of an overpressure nozzle used inthe counterpart of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a two-sided turning device 10 in accordance with theinvention for drying a coated paper web W. The device comprises a device12 turning the running direction of the paper web, and a dryingapparatus 14 arranged in the running direction of the web after the webturning device.

The turning device comprises the actual turning device 16 on the firstside of the web, in the case shown in the Figure above the web, and acounterpart 18 for this device on the second side of the web. Theturning device 16 comprises six blow nozzles 20 which, in the case ofthe Figure, are overpressure nozzles of the so-called Float type of theapplicant. The Float nozzles are symmetrical nozzles, from thelongitudinal slots on both edges transverse to the web of the carriersurface of which blows are directed against each other and against theweb, forming an overpressure zone between the nozzle and the web, andturning the running direction of the web about 70-80 degrees, in thecase shown in the Figure. The turning device 16 of the Figure has itsown air system with air supply channels 22 for bringing make-up air tothe turning device. In the turning device, air in the machine room,exhaust air from the airborne web-dryer, circulating air, or a mixtureof these, for example, may be used as blowing air.

The counterpart 18 has likewise six overpressure nozzles 24 arranged onthe second side of the web exactly opposite to the nozzles 20 of theturning device. The counterpart may have an air supply system of its ownwith air supply channels 26, as is shown in broken lines in FIG. 1. Thecounterpart may also have its own exhaust or return air system withexhaust air channels 28, into which air blown against the web isabsorbed, as is also shown in broken lines in FIG. 1. However, thesupply and discharge of air in the counterpart may advantageously bearranged through the drying apparatus, as is explained below.

The drying apparatus 14 is an airborne web-dryer with separate airborneweb-dryers or airborne web-dryer units 30 and 30′ on both sides of theweb. The upper airborne web-dryer unit 30 is combined with the turningdevice 16 under the same housing structure 32. However, the turningdevice is separated from the airborne web-dryer unit by a partition 39.The said airborne web-dryer unit 30 above the web has its own air supplysystem with air supply channels 34. The airborne web-dryer unit 30 alsohas its own exhaust air system with exhaust air channels 36. From theturning device 16, air is transferred along with the web to the upperairborne web-dryer unit, as is in an exemplary way shown with arrow 38,and from there onwards into the exhaust air channel 36. The necessaryamount of make-up air is brought to the turning device.

The lower airborne web-dryer unit 30′ is connected in a similar way tothe counterpart 18 with a common housing structure 40. Exhaust air fromthe counterpart is arranged to flow into the exhaust air channel 36′ ofthe airborne web-dryer unit 30′. Supply air, i.e. pressurized blowingair is led to the counterpart through the air supply channel 34′ of theairborne web-dryer unit 30′. The supply air systems for the airborneweb-dryer 30′ and the counterpart 18, as well as the exhaust airsystems, may be separated from each other by a partition 42 restrictingthe flow, which is provided with an adjustable damper 44 or a similarelement, as is shown in broken lines in FIG. 1, with which the supply ofair of the counterpart may be adjusted separately from the air flows ofthe airborne web-dryer unit.

In the airborne web-dryer units 30, 30′, the floating or blow nozzles 46and 46′ are interlaced so that the web runs in a sine-wave form throughthe straight airborne web-dryer section.

FIG. 1 also indicates the pressure measurements for the control systemof the turning device. The pressure sensor 48 arranged into the blownozzle 20 of the actual turning device 16 measures the nozzle pressureP_(SP) of the nozzle. The pressure sensor 50 arranged between thenozzles 20 of the turning device measures the pad pressure P_(KL) of theturning device.

The pressure sensor 56 arranged between the nozzles 24 of thecounterpart may respectively be used for measuring the possibleunderpressure or overpressure P_(VK) in the counterpart.

In FIG. 1, the small arrows indicate how the blows from the nozzles 20and 24 arranged on both sides of the web blow against each other,forming a local overpressure between the nozzle carrier surfaces 52 and54 and the paper web on both sides of the web. These local overpressureshave a stabilizing effect on the paper web and improve the runnabilityand controllability of the web.

FIG. 3 shows an enlargement of an overpressure nozzle 24 of the U.S.Pat. No. 4,384,666 used in a counterpart of the invention. The arrowsindicate the direction of the blows from the carrier surface 54 towardsthe web.

The control system for a two-sided turning device in accordance with theinvention is shown in more detail in FIG. 2. It may be seen from FIG. 2,that the measuring results from the differential pressure instruments 48and 50 are led to the control device 58 with which it is possible tocontrol the blower 60 feeding air into the air supply channel 22 of theturning device 16.

Also the air supply channels 34 and 34′ and exhaust air channels 36 and36′ in the airborne web-dryer units 30 and 30′ may be seen in theFigure. FIG. 2 shows the alternative in which both the supply air andthe exhaust air arrangement of the counterpart are connected to theairborne web-dryer unit 30′.

The invention is above described in an exemplary way, referring mainlyto one embodiment. The purpose is by no means to restrict the inventionto this embodiment only, but the invention is intended to be widelyapplied within the scope of protection defined by the enclosed claims.

What is claimed is:
 1. A method for drying a coated paper web in adevice including, in a running direction of the web, a web turningdevice arranged on a first side of the web and provided with blownozzles, a counterpart arranged on a second side of the web in an areaof the turning device, and web drying devices arranged on the first andsecond sides of the web, the method comprising: (a) turning the runningdirection of the web in a non-contacting way by blows generated by theblow nozzles; (b) generating blows in an area of the turning device onthe second side of the web with overpressure nozzles of the counterpart,thereby directing a pushing force against the web, pushing the web; and(c) thereafter drying the web in a non-contacting way with the webdrying devices.
 2. A method according to claim 1, wherein step (b) ispracticed by directing the blows in the area of the turning device onthe second side of the web against the blow nozzles of the turningdevice arranged on the first side of the web.
 3. A method according toclaim 1, wherein the overpressure nozzles are used for blowing hot air,the temperature of which is 150-400° C., and the speed of which is 40-80m/s.
 4. A method according to claim 1, wherein the overpressure nozzlesare used for blowing hot air, the temperature of which is 100-450° C.,and the speed of which is 20-100 m/s.
 5. A method according to claim 1,further comprising controlling a distance H between a nozzle surface ofthe blow nozzles of the turning device and the web by adjusting aninternal pressure P_(SP) of the blow nozzles of the turning device and apad pressure P_(KL) between the turning device and the web in accordancewith the following formula: $H = {{a\frac{P_{SP}}{P_{KL}}} + b}$

in which H is the distance (mm) between the nozzle surface and the web;P_(SP) is the internal pressure (Pa) of the blow nozzles; P_(KL) is thepad pressure (Pa) between the turning device and the web; a is anamplification coefficient for the machine; b is a difference variablefor the machine.
 6. A method according to claim 1, further comprisingadjusting a web tension T by utilizing pad pressure P_(KL) between theturning device and the web, and a pressure P_(VK) between thecounterpart and the web, in accordance with the following formula:T=C*[P_(KL)(r+h)−k_(VK)P_(VK)(r+h)+Mv²] in which C is an amplificationcoefficient relating to the machine in question within a range of0.7-1.4; r is a radius (m) of the turning device; h is a distance (m)between the turning device and the paper web; T is the tension (N/m) ofthe paper web; M is a grammage (kg/m²) of the paper web; v is a speed(m/s) of the paper web; P_(KL) is the pad pressure (Pa) between theturning device and the web; P_(VK) is the pressure (Pa) between thecounterpart and the web; k_(VK) is a parameter constant within a rangeof 0.6-1.
 7. A method according to claim 6, wherein the amplificationcoefficient C is 1.0, and wherein the parameter k_(VK) is 0.8.
 8. Amethod according to claim 1, wherein the drying devices include airbornedrying units provided with exhaust air channels and arranged on thefirst and second sides of the web, the method further comprisingabsorbing at least one of blowing air from the blow nozzles of theturning device and blowing air from the overpressure nozzles of thecounterpart into the exhaust air channels of the airborne web dryingunits.
 9. A method according to claim 1, further comprising dischargingair from the turning device primarily into the web drying devicesarranged after the turning device.
 10. A method according to claim 1,wherein step (a) is practiced by controlling the web run by adjusting apressure prevailing in the counterpart.
 11. An apparatus for drying acoated paper web comprising, in a running direction of the web: a webturning device arranged on a first side of the web and provided withblow nozzles, the blow nozzles generating blows that turn the runningdirection of the web to be dried in a non-contacting way; a counterpartarranged on a second side of the web in an area of the turning device,the counterpart including overpressure nozzles arranged on the secondside of the web, the overpressure nozzles producing blows that generatea pushing force on the second side of the web, pushing the web; and webdrying devices arranged on the first and second sides of the web, theweb drying devices including floating nozzles for non-contacting dryingof the web.
 12. An apparatus according to claim 11, wherein at least oneof the blow nozzles and the overpressure nozzles are symmetricaloverpressure nozzles with air flow from slots on both edges from carriersurfaces of the nozzles against each other, thus forming an overpressurezone between the nozzles and the web.
 13. An apparatus according toclaim 11, wherein the web drying devices are airborne web-dryer unitsarranged on the first and second side of the web and provided withexhaust air channels for discharging blowing air from a space betweenthe floating nozzles and the web, wherein a space between the turningdevice and the web is in contact with the exhaust air channel of theairborne web-dryer units on the first side of the web for dischargingblowing air from the turning device, and wherein a space between thecounterpart and the web is in contact with the exhaust air channel ofthe airborne web-dryer units on the second side of the web fordischarging air blown toward the web by the overpressure nozzles of thecounterpart.
 14. An apparatus according to claim 11, wherein the blownozzles of the turning device are provided with a first pressure sensorfor measuring an internal pressure P_(SP) of the blow nozzle, andwherein the turning device is provided with a second pressure sensor formeasuring a pad pressure P_(KL) between the turning device and the web,the apparatus further comprising control elements for calculating adistance H between the nozzle surface of the turning device and the weband for adjusting the distance to a desired level on the basis of valuesfrom the pressure sensors, in accordance with the following formula:$H = {{a\frac{P_{SP}}{P_{KL}}} + b}$

in which H is the distance (mm) between the nozzle surface and the paperweb; P_(SP) is the internal pressure (Pa) of the blow nozzles; P_(KL) isthe pad pressure (Pa) between the turning device and the web; a is anamplification coefficient for the apparatus; b is a difference variablefor the apparatus.
 15. An apparatus according to claim 14, furthercomprising an air channel and a blower that direct air into the blownozzles of the turning device the control elements comprising elementsfor controlling an amount of air to be fed into the blow nozzles.
 16. Anapparatus according to claim 11, wherein the turning device is providedwith a first pressure sensor for measuring a pad pressure P_(KL) betweenthe turning device and the web, and wherein the counterpart is providedwith a second pressure sensor for measuring a pressure P_(VK) betweenthe counterpart and the web, the apparatus further comprising controlelements for adjusting a tension T of the web on the basis of valuesfrom the pressure sensors, in accordance with the following formula:T=C*[P_(KL)(r+h)−k_(VK)P_(VK)(r+h)+Mv²] in which r is a radius (m) ofthe turning device; h is a distance (m) between the turning device andthe paper web; T is the tension (N/m) of the paper web; M is a grammage(kg/m²) of the paper web; v is a speed (m/s) of the paper web; P_(KL) isthe pad pressure (Pa) between the turning device and the web; P_(VK) isthe pressure (Pa) between the counterpart and the web; k_(VK) is aparameter constant within a range of 0.6-1.
 17. An apparatus accordingto claim 16, wherein the parameter k_(VK) is 0.8.
 18. An apparatusaccording to claim 11, further comprising a housing structure covering apart of the counterpart facing away from the web, the housing structurecomprising an exhaust air channel that absorbs air blown towards thepaper web from a space between the web and the overpressure nozzles. 19.An apparatus according to claim 11, further comprising a common housingstructure covering the web drying devices arranged on the second side ofthe web the housing structure including an exhaust air channel thatabsorbs drying air and air blown through the overpressure nozzles in thecounterpart.
 20. An apparatus according to claim 11, wherein the turningdevice comprises 3-15 blow nozzles arranged facing the first side of theweb and wherein the counterpart comprises 3-15 overpressure nozzlesarranged facing the second side of the web, a majority of theoverpressure nozzles being arranged to blow towards the web to points onthe second side of the web corresponding to points on the first side ofthe web at which the blow nozzles are facing.
 21. An apparatus accordingto claim 11, further comprising a housing structure covering the turningdevice and the web drying devices arranged adjacent and wherein apartition is provided between the turning device and the web drivingdevices for maintaining pad pressure in the turning device.